November 18, 2013

(Austin – November 18, 2013) In most markets, rooftop solar panels are promoted as a way for electric utility customers to reduce their reliance on fossil fuel power and — eventually — save money on their electric bill. According to a new PSR Analytics report from Texas-based energy research firm Pecan Street Research Institute, residential solar systems, and particularly west-facing rooftop systems, may also act as a fairly impactful peak demand reduction device for utilities struggling to meet afternoon demand in hot summer months.

The report draws on original data from a research trial of residential solar panel performance and of how much of the rooftop generated energy was used inside the home (as opposed to sent back to the grid). The 50 single-family homes analyzed in the report are a randomly selected subset of the 175 homes with rooftop solar PV participating in the research trial. Roughly half (24) the homes studied have south-facing solar arrays, 14 have west-facing arrays, and 14 have a combination of west- and south-facing arrays. All homes are located in Austin, Texas.

“Residential solar systems have understandably raised concerns about their impact on electric reliability,” said the report’s lead author, Pecan Street CEO Brewster McCracken. “These findings suggest that rooftop solar systems can produce large summer peak reductions that benefit utilities and customers alike without requiring customers to change their behavior or sacrifice comfort.”

One surprise from the study was that west-facing solar systems in the sample actually produced more electricity than south-facing systems (on both an actual and a normalized basis). During summer peak demand hours (3-7 pm), the gap was even more pronounced: west-facing systems produced 49 percent more electricity during those hours than did south-facing systems. As a result, the report observes, utilities that offer residential rooftop solar rebates may want to extend rebate eligibility to west-facing systems and even offer higher rebate levels than is provided to south-facing systems.

The analysis focused explicitly on a single period of the year: June 1 – August 31, 2013. The summer months have more daylight hours and higher levels of seasonal electricity use for air conditioning (in areas with such demands). Summer months present very different home energy use and solar generation profiles than other seasons.

Key findings from the report include:

Counting only the electricity generated by a rooftop solar system that is actually used in the home (and therefore not counting electricity that was sent to the grid because it could not be used in the home), homes averaged a 58 percent peak demand reduction for electricity from the grid. South-facing solar systems cut peak demand from the grid by 54%, while west-facing systems reduced their homes’ peak demand from the grid by 65%.

During peak hours, homes used 80% of the power generated from the rooftop systems and returned 20% to the grid. In the homes with south-facing systems, 78% of the power generated was used in the home; 22% was returned to the grid. In homes with west-facing systems, 84% was used in the home; 16% was returned to the grid.

Over the course of the full day, 64% of the energy generated by the rooftop systems was consumed on-site; 36% was returned to the grid.

Over the course of the full day, and not including surplus energy returned to the grid, the solar systems provided 36% of the average power used per home. Nearly a third (32%) of the power was generated during peak demand hours.

About PSR Analytics and the Pecan Street Research InstitutePSR Analytics reports are produced quarterly by the Pecan Street Research Institute. Reports cover research findings from the institute’s research trials, including HVAC use, electric vehicle charging, rooftop solar PV impacts, transformer impacts from new technologies and home appliance use. The analyses are based on Pecan Street’s extensive research network of 1,000 volunteer houses, town homes and apartments in Texas, California and (starting later this fall) Colorado. PSA Analytics reports are available exclusively to Pecan Street’s industry advisory council and research clients.

The Pecan Street Research Institute is a non-profit university-based scientific research institute. The Institute’s public interest research focuses on advancing understanding and solutions addressing utility system reliability, climate change, renewable energy integration and customer needs and preferences. The Institute is the nation’s most significant creator of original customer energy use research data available to the research community. Pecan Street’s specific research expertise consists of creating, managing, protecting, analyzing and responsibly sharing the highest quality original research data on how customers use electricity, natural gas and water in their homes and businesses.

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